Slow Speed Grinders Silver City NM

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Slow-Speed Grinders - Tools - American Woodworker

Slow-Speed Grinders

These 10 machines are designed to save your tools from overheating: Do they measure up?

by Tom Caspar and Jason McNellis

If you love hand tools, chances are you’ll need a grinder. When a blade becomes very dull, chipped or nicked, grinding is the fastest way to renew its bevel.

But have you ever seen a tool that has been overheated by a grinder? When an edge becomes too hot, it loses its temper and turns bright blue. The only solution is to grind off the softened section and start again.

Slow-speed grinders are designed to prevent this problem. They run approximately 50 percent slower than high-speed machines, which are built for general-purpose metalworking. You can successfully grind woodworking tools with a high-speed grinder if you’re extremely careful, but we much prefer slow-speed machines. Many have variable speed for quick stock removal as well.

We examined 10 slow-speed grinders priced from $80 to $300. Before you buy, you should know four things about a grinder: how slow it is, whether it has good tool rests, whether it has at least one soft-bond wheel and whether it comes with a wheel dresser.

Slow Speed Has a Wide Range

Slow is slow, right? Not exactly. The wheel’s surface travels much faster on some machines than on others, and that makes quite a difference in heat buildup. Higher surface speeds create more friction and more heat. A wheel’s surface speed, measured in surface feet per minute (sfpm), depends on two factors: the motor’s revolutions per minute (rpm) and the wheel’s circumference.

You might think all slow-speed grinders spin at the same speed, but they don’t. At ther slowest settings, they range from 1,120 to 2,000 rpm (see Profiles, pages 68 and 69). Most are in the range of 1,725 to 2,000 rpm.

Slow-speed grinders have 6- or 8-in.-dia. wheels. The distance around a 6-in. wheel is about 1.5 ft.; the distance around an 8-in. wheel is about 2 ft. A 6-in. wheel’s surface travels 33 percent slower than an 8-in. wheel’s surface, if both wheels rotate at the same speed.

Combine both factors, speed and circumference, and you get the sfpm number, the one number that tells you the most about heat buildup on any particular grinder. It’s easy to figure out: Speed (rpm) multiplied by a wheel’s circumference equals surface speed (sfpm). We’ve done the math for each machine.

It’s no surprise that 6-in. machines grind at cooler temperatures than 8-in. machines do. The minimum sfpm rate for 6-in. grinders varies from 1,760 to 3,140 sfpm, for 8-in. grinders, from 3,610 to 3,770 sfpm. Both work well, but you must exercise a lighter touch and quench more often when using an 8-in. grinder.

Have a Good Rest

Tool rests on most slow-speed grinders don’t offer the precision, adjustability and convenience we want for woodworking tools. Only one ...

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